In industries such as petroleum refining and ethylene production, aqueous caustic washing is sometimes employed to improve the quality of the product and/or aid in the refining process. The caustic washings are done to remove, for example, sulfidic and/or acidic components from the relevant hydrocarbon streams. Aqueous spent caustic streams from these treatments may contain high chemical oxygen demand (COD) and other contaminants, such as sulfides, mercaptans, naphthenates, cresylates, and emulsified hydrocarbons, for example. The aqueous spent caustics often have high pH concentrations, for example, pH concentrations of about 13 or above. Environmental and safety considerations require treatment of spent caustics before discharge to the environment.
To reduce COD concentrations in the spent caustic, biological treatment of the wastewater is widely practiced. The wastewater is commonly treated with activated sludge such that dissolved and suspended organics, for example, are acted upon by bacteria during a sludge residence time within a bioreactor (e.g., an aerated treatment tank). The odorous and reactive nature of spent caustic, however, often precludes the use of biological treatment alone as the sole method of treatment—even with dilution of the spent caustic. For this reason, another treatment, e.g., wet air oxidation, is utilized in combination with (e.g., upstream of) biological treatment for the treatment of spent caustic.
Wet air oxidation (WAO) is a well-known technology for treating process streams, and is widely used, for example, to destroy oxidizable contaminants in wastewater, such as the aforementioned spent caustic streams. The process involves aqueous phase oxidation of undesirable constituents by an oxidizing agent, generally molecular oxygen from an oxygen-containing gas, at elevated temperatures and pressures. By way of example, the process may convert organic contaminants to carbon dioxide, water, and biodegradable short chain organic acids, such as acetic acid. Inorganic constituents, including sulfides, mercaptides, and cyanides, can also be oxidized. In the context of spent caustic, WAO detoxifies the spent caustic by oxidizing sulfides and mercaptans to sulfate and breaking down toxic naphthenics and cresylics.
One issue with the biological treatment of spent caustic streams, however, is that total dissolved solids (TDS) concentrations in the spent caustic are typically incompatible with biological treatment—even after wet air oxidation. The TDS mainly include salts, and also may include dissolvable organics. If the TDS/salt concentration is too high, the spent caustic may cause a decrease in biological treatment efficiency, which could result in high effluent concentrations of soluble COD and soluble nitrogen, and decreased biological solids settling. In addition, the TDS/salt concentration may cause significant osmotic pressure increases in the bioreactor, which could further result in shutdown of the bioreactor. For these reasons, the spent caustic stream is often added to a very large biological treatment system which results in significant dilution of the spent caustic. The dilution fluid utilized, however, significantly adds materials, costs, handling, and volume to the overall system.
Moreover, if the spent caustic is desired to be reused (after treatment) for purposes such as boiler feed water or the like, salts will need to be removed therefrom. A common salt removal method is reverse osmosis. However, significant dilution of the caustic stream to allow for biological treatment and reverse osmosis also significantly increases the volume of the stream, and thus thereby significantly increases the size of the associated reverse osmosis unit(s) required and volume of fluid to be processed. Reverse osmosis also generally requires a pretreatment step, which would also increase the system footprint relative to a system which treats the spent caustic stream without dilution.